1. Field of the Invention
The present invention relates to sensors for measurement of fluid level. The measured fluid may be contained in a sump, reservoir, process vessel, holding tank, or other man made or natural fluid container. A fluid container is herein called the vessel. An example of such a vessel is a holding tank for a marine use. At least a part of at least one vessel wall is fabricated from an electrically non-conductive material. The terms electrically non-conductive, insulating, and dielectric are used here interchangeably.
More particularly, the present invention relates to a capacitive level sensor that measures fluid level in the vessel without the use of a float, and functions based on the permitivity of the measured fluid being substantially different from that of air, the sensing element of the capacitance level sensor being electrically insulated from the measured fluid.
The capacitive level sensor of the present invention measures fluid level while the sensing element is mounted on, or embedded within, an outside wall of the vessel. Alternatively, the present invention also teaches the embedding of the sensing element into a wall of a device that utilizes fluid level information, such as a bilge pump.
2. Description of the Prior Art
Prior art capacitive level sensors have been built which depend on the permitivity of a measured fluid. U.S. Pat. No. 4,924,702 teaches a capacitive level sensor that is inserted into a reservoir containing a liquid. Conversely, the present invention teaches the design of a sensor that is not immersed into the measured fluid, but is instead mounted on the outside of a wall of a vessel that contains the measured fluid. The separation of the sensor electrodes from the sensed fluid in the present invention allows the sensor to operate corrosion-free while measuring a corrosive fluid, such as seawater. Even when an immersed sensor is anodized as taught in the prior art (U.S. Pat. No. 6,564,658), or otherwise coated with a compatible material, corrosion and electrical conductivity can remain a problem due to inconsistency or pinholes in the coating or due to deterioration of the coating with age and use. The present invention also allows for easy addition of a level sensor to an already-installed vessel. An alternative embodiment of the present invention teaches embedding the sensing element within the vessel wall. Still another embodiment of the present invention teaches embedding the sensing element into an electrically non-conductive member of another device that utilizes fluid level information, such as a bilge pump, located within the vessel. In all embodiments of the present invention, direct contact of the sensing element with the measured fluid is avoided.
U.S. Pat. No. 6,164,132 teaches a capacitive liquid level indicator that incorporates dual capacitive sensor arrays, disposed on opposite sides of a dielectric substrate. In order to expose the two sides of the sensor array assembly to the measured liquid, the complete sensor array assembly is immersed into the measured liquid, contrary to the present invention.
U.S. Pat. No. 4,142,415 teaches a device for continuously measuring the liquid level in a container, wherein a capacitive sensor is disposed in the liquid. This is also contrary to the present invention, as the present invention teaches the separation of the sensing element from the measured fluid by a dielectric material.
U.S. Pat. Nos. 6,293,142 and 6,564,658 teach electromagnetic sensors that use special “slow wave structure” electrode configurations, called electrodynamic elements, that, when driven by a radio frequency generator, produce a “slowed wave”. The wave is slowed due to the electromagnetic cooperation (related to the travel time of the electromagnetic wave between adjacent conductors of the electrodynamic element) among adjacent conductors arranged in series with spacing. The ratio by which the wave is slowed, on comparison to the natural speed of an electromagnetic wave in the materials used, is called the “slowing factor”. This measurement technique is, according to column 2, line 67 of U.S. Pat. No. 6,564,658 and column 2, line 60 of U.S. Pat. No. 6,293,142, “ . . . contrary to the capacitance and inductance sensitive elements”.
The present invention utilizes a capacitance sensitive element, herein called the sensing element. The present invention does not use a slowed wave related to the propagation time between adjacent conductors, but relies instead on the positioning of electrode conductors with respect to one another and optionally, also with respect to a guard or shield conductor, in order to provide control of the shape and size of the sensing field of a capacitive sensor. In the present invention, the field size is made large enough to penetrate a dielectric wall of the vessel, or a dielectric wall of a device that utilizes fluid level information, in order to measure the level of fluid within the vessel.